well, with the fists you have the ability to knock tanks down. Thats the mayor shiny glittery + on fists. Basically, if you think your gonna send your bezerker down the enemy who has tanks, or even more important, walkers, take the fist.
If you want to risk the chance that your unit never comes in touch with a walker, take the power weapon.
If you want to risk the chance that your unit never comes in touch with a walker, take the power weapon.